Recently, in order to perform wavelength multiplexing transmission of a signal light with a speed as high as 40 gigabits per second (Gbps) and 100 Gbps at high density, investigations have been actively made of an optical modulation system having a high spectral density. Moreover, since vigorous research and development of coherent reception techniques have been made, realization of a receiver applying a 16 QAM system, which is a multilevel modulation system using more multilevels than a differential quadrature phase shift keying (DQPSK) system that has heretofore come into practical use, becomes realistic, and investigations thereof have been made.
Under such a background, recently, as one form of the optical modulator according to the 16 QAM system, a configuration has been proposed in which two QPSK modulators are connected in parallel and the signal lights modulated by respective QPSK modulators are multiplexed so that a power ratio thereof becomes 4:1, (for example, refer to T. Sakamoto, A. Chiba and T. Kawanishi, “50-Gb/s 16 QAM by a quad-parallel Mach-Zehnder modulator”, ECOC 2007, PD2.8).
Incidentally, regarding the configuration of the optical modulator according to the 16 QAM system, a phase difference at the time of multiplexing output lights from the two QPSK modulators may deviate from an ideal value of n×90° (n is an integer) due to individual differences of the QPSK modulators, DC drift, wavelength dependency, temperature dependency, deterioration with age or the like. Moreover, the power ratio of a component modulated by each QPSK modulator occupying the output light may deviate from the ideal value of 4:1 due to individual differences of the QPSK modulators, wavelength dependency, temperature dependency, deterioration with age or the like.
Specifically, FIG. 25 is a diagram schematically illustrating an aspect where a constellation of a 16 QAM signal light (expressing a relation between phase difference and amplitude for each symbol in a rectangular coordinate system, with an in-phase component I being plotted on the horizontal axis and a quadrature component Q being plotted on the vertical axis) is changing due to a deviation of the phase difference in the output light of the respective QPSK modulators. Respective symbols (black dots in the drawing) of the 16 QAM signal light are in an arrangement as illustrated in the upper part of FIG. 25 when the phase difference is in the ideal state (n×90°), where distances between respective symbols become equal. On the other hand, when the phase difference deviates from the ideal state, as illustrated in the lower part of FIG. 25, the arrangement of the respective symbols rotates about a central point of four symbols in each of the first to the fourth quadrants. Actually, rotation about an origin of the rectangular coordinate system can occur as well. However regarding this, by rotating the coordinate system itself and redefining it, the constellation state can be sufficiently expressed without impairing generality. Therefore, rotation about the origin is not considered here.
Moreover FIG. 26 is a diagram schematically illustrating an aspect where the constellation of the 16 QAM signal light is changing due to a deviation of the power ratio of the component modulated by each QPSK modulator and occupying the output light. When the power ratio is in the ideal state (4:1), respective symbols of the 16 QAM signal light are in the arrangement as illustrated in the upper part of FIG. 26, where the distances between respective symbols become equal. On the other hand, when the power ratio is larger than 4:1, as illustrated in the middle part of FIG. 26, a gap between the four symbols in the first to the fourth quadrants becomes narrow. Moreover, when the power ratio is smaller than 4:1, as illustrated in the lower part of FIG. 26, the gap between the four symbols in the first to the fourth quadrants becomes wide.
When a change occurs in the symbol arrangement of the 16 QAM signal light due to a deviation from the ideal state of the phase difference or the power ratio of the output light of the respective QPSK modulators as described above, the distances between respective symbols are not equal. Therefore, the receiver compatible with the 16 QAM system may not operate correctly, and it is assumed that an error tends to occur in the receiver, influenced by noise light.
However, a technique for monitoring the deviation of the phase difference and the deviation of the power ratio between phase modulated lights, multiplexed in the optical modulator according to the 16 QAM system as described above, has not yet been proposed. Moreover, a specific technique teaching how to adjust the deviation and stabilize the modulator in the ideal state has not yet been proposed.